The present invention relates to a color image forming apparatus in which a toner image is formed on an image forming body by an electrophotographic method and the toner image is transferred onto a transfer sheet to obtain a color image.
As an image forming method by which a color image is obtained using an electrophotographic method, the following method is used: latent image formation and development corresponding to the number of separation colors of an original image are repeated on the image forming body; color toner images are superimposed on the image forming body; and after that, they are transferred so that a color image is obtained, as disclosed in, for example, Japanese Patent Publications Open to Public Inspection No. 75850/1985, 76766/1985, 95456/1985, 95458/1985, and 158475/1985.
As a color image forming apparatus to which the image forming method is applied, by which the latent image formation and development corresponding to the number of separation colors of the original image read in by a color scanner are repeated on the image forming body, the following apparatuses are used. The first example of them is an image forming apparatus in which exposing units and developing units corresponding to the number of separation colors (for example, three colors of yellow, magenta, and cyan, or four colors including black) are provided around a belt-shaped image forming body, and the second example is an image forming apparatus in which exposing units and developing units corresponding to the number of colors are provided around a drum-shaped image forming body. The color image forming apparatus provided with the belt-shaped image forming body will be described as follows.
The color image forming apparatus is structured in the following manner: a tension roller is provided to the belt-shaped image forming body on which a photoconductive body is coated or deposited; the belt-shaped image forming body is conveyed so that the surface of the belt-shaped image forming body is maintained always at a constant position when the belt-shaped image forming body is rotated, maintaining slidable contact with a guide member, which is a reference position, by tension of the belt due to pressure contact of the tension roller; an image forming means composed of a charger, exposing units, and a plurality of developing units in which different color toners (yellow, magenta, cyan, black) are contained, is provided around the belt-shaped image forming body; and the image forming means is provided around the rotating belt-shaped image forming body maintaining a constant gap therefrom.
Recently, the following color image forming apparatus is widely known in which: a latent image is formed by a beam into which color signals corresponding to yellow (Y), magenta (M), cyan (C), and black (K) are modulated, using a laser writing unit as an exposing unit by which a document image is rotatively scanned using a polygonal mirror by a laser beam outputted from a laser light source which is modulated or turned on and off, and the image forming body is exposed; the latent image is visualized by color toner development by the developing unit corresponding to each color; and each image is successively transferred onto a transfer sheet so that a multi-color, or full color image is obtained (Japanese Patent Publication Open to Public Inspection No. 156157/1986, etc.).
In the color image forming apparatus, the following method has been considered: a registration mark is formed on the image forming body, for example; the registration mark is detected by a sensor; a plurality of exposing units start exposing successively according to the detection; and thereby each latent image formation is started from the same point on the image forming body.
However, int he color image forming apparatus, when the registration mark formed on the belt-shaped image forming body is read out by a single sensor or a plurality of sensors, and a timing to start exposing from the exposing units is determined, a space between the sensor and the exposing unit and an arrangement space of a plurality of exposing units are required to be determined with exact accuracy (about .+-.0.01 to 0.1 mm) in this method, and therefore, it is actually difficult to position an arrangement space of a plurality of sensors or exposing units with mechanical accuracy (at the time of application of the present invention, the limitation is about .+-.0.3 to 0.5). Further, when the exposing unit is disassembled for maintenance, it is extremely difficult to assemble the unit at exactly the same position again.
Further, in the color image forming apparatus in which toner images are superimposed on the image forming body so that a color toner image is formed as described above, when an exposure starting position of a plurality of exposing units on the belt-shaped image forming body is not set within the unit of a pixel, for example, less than about 80 .mu.m, quality of a color toner image is lowered. Especially when the belt-shaped image forming body is used, positioning control is difficult compared with a drum-shaped image forming body.
Further, conventionally, a method in which a sensor for exclusive use is used for position detection, has been accepted for detecting a pattern image provided previously on the image forming body (a photoreceptor) or transfer sheet, or a toner image formed on the photoreceptor or transfer sheet. In this case, since the sensor for exclusive use is provided to control the apparatus, it is disadvantageous in terms of space and cost, and further sufficiently accurate positioning is not obtained, so that the image quality is lowered.
Further, in an optical writing system, an optical system disclosed in Japanese Patent Publication Open to Public Inspection No. 95361/1983 is widely known for the purpose of compactness. However, in this case, since the scanning direction of the laser beam is opposite to the writing direction, a pattern image on the photoreceptor or a transfer sheet is necessary at both sides of the scanning direction.